Lungs have rarely been reported to be affected by any side effects of BCG therapy. Interstitial pneumonitis, though, is known to occur under such circumstances, but its pathogenesis is still debated between an infectious and a hypersensitivity mechanism. We report here 3 cases of pulmonary complications of BCG therapy evaluated by bronchoalveolar lavage (BAL). Cellular data obtained from all 3 patients were characterized by a markedly increased alveolar lymphocytosis. The T4/T8 ratio was elevated compared with that in normal subjects and with the T4/T8 ratio of circulating lymphocytes. Furthermore, alveolar lymphocytes were highly sensitized to PPD, as evaluated by their proliferation and their production of interleukin 2 in the presence of PPD. Mycobacteria were not found in the 3 patients. We conclude that interstitial pneumonitis occurring during BCG therapy could be explained by a hypersensitivity phenomenon, leading to an intense immune and lymphocyte-mediated response within involved organs.